In refrigerant system devices used for refrigeration, freezing, air conditioning and the like, chlorine element-free alternative refrigerants have begun to be employed as a working fluid in terms of the fact that the ozone layer destruction due to chlorofluorocarbon refrigerants used as the working fluid contributes to the global environmental problem. As such alternative refrigerants, for example, hydrofluorocarbon (HFC) type alternative chlorofluorocarbon refrigerants comprising hydrogen, fluorine and carbon are being used.
Also, from the viewpoint of preventing the global warming, energy savings are desired for electric and electronic equipment, and energy-saving measures are being investigated for the respective pieces of the equipment. In compressors used for refrigerant system devices, the inverter control system permitting the precise control of rotators is increasingly being employed in order to improve the operation efficiency. As a result, high frequency components in the leakage current are increased in the electrical insulation part, so that the energy-saving efficiency is reduced by the increased leakage current. In order to prevent this, it is desired to improve the insulating property of the insulating material.
Among electrically insulating materials, plastic films have high insulating property, and thus are applied to electronic and electric equipment as those parts and members which are required to be reliable. For example, they are applied to: an insulating material for covering a cable; a printed circuit board; and a slot insulation part of a rotator. Additionally, they are applied to electronic parts such as a film capacitor. In the case of the rotator, plastic films have been widely used for: a slot insulation part, which is a groove provided in an iron core for housing a field coil; a slot spacer; a wedge; an outer jacket and the like. Among such insulating plastic films, a polyester film such as polyethylene terephthalate (PET) is being used as a general-purpose film in terms of its excellent insulating property, heat resistance, moldability, cost effectiveness and the like.
Herein, FIG. 3 shows a schematic transverse sectional view of an iron core included in a rotator of a compressor, and FIG. 4 shows a schematic partial perspective view of the iron core. As shown in FIGS. 3 and 4, an iron core 1 is provided with a plurality of slots 5. A coil will be wound around the slot 5.
However, in these days of increasing numbers of pieces of equipment employing high frequency, high frequency components in the leakage current are increasing; therefore, a method is recently being sought after, by which the dielectric constant of the insulation part is decreased to lower the electrical capacity and thereby reducing the leakage current. For example, one method is available, by which the thickness of the insulation part is increased in order to lower the electrical capacity; however, the use of this method for the insulation of the slot reduces the utilization of an iron core slot, as a result of which the important characteristics of the rotator, such as the size, weight and efficiency, must be sacrificed.
In order to solve this problem from the viewpoint of the insulating material, materials having a low dielectric constant may be selected. The specific dielectric constants of typical plastics are as follows: polyethylene terephthalate (PET), which is an engineering plastic with excellent heat resistance, has a dielectric constant of about 3.1; polyimide (PI) has a dielectric constant of about 3.3; polyethylene (PE) with a low dielectric constant has a dielectric constant of about 2.3; and polytetrafluoroethylene (PTFE) with the least dielectric constant has a dielectric constant of about 2.1.
In general, the dielectric constant of a material is determined by the molecular structure of the material and the dielectric constant depends upon the material, so that each material has its inherent dielectric constant. Accordingly, the bulk dielectric constant of plastic has its limit. In addition, it is necessary that other practical properties such as processability and heat resistance also be satisfied. For example, although a fluorocarbon resin such as PTFE, which is excellent in term of the low dielectric constant, has sufficient heat resistance, it has problems of high cost and poor moldability. Also, an olefin resin such as PE has a heat resistance temperature of up to around 100° C., and therefore does not have sufficient heat resistance to be applied to the rotator.
Moreover, as a method for reducing high frequency components in the leakage current from the viewpoint of the material, a method which employs a porous polyester film as an electrical insulating material is disclosed in JP-A-9-100363. This method forms a void inside the film to reduce the density of the film, thereby achieving an effect of reducing the dielectric constant. Then, the use of the low dielectric constant polyester film disclosed in JP-A-9-100363 as the electrical insulating material for the rotator produces an effect of reducing the leakage current.
However, in the case where this rotator is installed in a compressor used for a refrigerant system device or the like, the working fluid such as a refrigerant should be taken into account when considering the effect. Particularly, since the rotator is disposed in a working fluid in the case of a closed-type compressor, the dielectric constant of the working fluid exerts an influence on the effect, unlike the case where the atmosphere is air (having a specific dielectric constant of 1). For example, when an HFC type alternative chlorofluorocarbon refrigerant is used as the refrigerant of this working fluid, the dielectric constant of the atmosphere in which the rotator is disposed is increased, since this refrigerant is polar. For this reason, even when the leakage current is decreased owing to the rotator employing a low dielectric constant plastic film for the insulation part, this effect is reduced because the working fluid, unlike the air, causes an increase in an electrical capacity of the compressor as a whole.
Therefore, it is an object of the present invention, for the purpose of realizing energy savings for a refrigerant system device and the like, to reduce the leakage current in a compressor used for the refrigerant system device thereby to improve the safety and reliability.